A short, genius, E-ranked main campus student who has the unique ability to do anything as long as she has seen it accomplished just once. She also has a rough time telling people apart; having to rely on smell and taste to differentiate people from one another.

Junichirou first met Reiko when she arrived at KEC as a one-day intern; set-up by Koyomi to give Reiko a taste of Junichirou’s “interesting” hobbies. She has come to see Junichirou as her master after gaining an appreciation of his said hobbies. However, Junichirou catches on to Reiko’s servant tendencies and commands her to stay in her room. Despite not knowing what happened initially, she finally gets it when she goes against Junichirou’s command to stay in her room to stop a cyber-attack on KEC; learning that she won’t be able to enjoy life if she always followed other people’s orders. She decides to return to school, wanting to follow Junichirou’s ideals.

She reappears later on hiding in Junichirou’s classroom from Shirakawa Nami who had been looking for Reiko to drag back to the main campus. Reiko had run away, because she doesn’t like the rules of the main campus, and is considered a burden for never being able to obtain any points to increase her E rank. Threatened by Arisu to become part of her prison system, Junichirou initiates a duel in basketball between Reiko and Nami. Reiko goes on to win the game thanks to a scheme set by Junichirou to have Reiko learn all of the branch student’s “unique” moves to which she put to use against Nami. Afterwards, she watches from afar as Junichirou makes his declaration to become principle.

Alongside Nami, who she is now good friends with, they form a sports team soon after known as White Princess, and join up with Junichirou as his first children after he and his five chosen students appear before them when they are threatened by Team Titan. Since then, she follows Junichirou along on his main campus escapades.